In general, a printed wiring board is produced by the steps of bonding an insulating substrate to a copper foil to form a copper clad laminate and forming a conductive pattern on the copper foil surface by etching. With increase in the recent needs for miniaturization and higher performance of electronic devices, high density mounting of components and high frequency of signal have progressed and therefore it is demanded that the printed wiring boards meet the requirements of refinement (fine pitching) of the conductive pattern and of adapting to high frequencies.
Corresponding to the fine pitching, a copper foil having a thickness of 9 μm or less, furthermore preferably 5 μm or less, is recently demanded. However, such an ultrathin copper foil has a low mechanical strength and accordingly there are such problems as breakage and generation of creases during the production of the printed wiring board. Accordingly, a carrier-attached copper foil, wherein a thick metal foil is adopted as the carrier and an ultrathin copper layer is electrodeposited on the carrier via a peeling layer between them, has been proposed. A generally adopted manner of use of the carrier-attached copper foil is such that the surface of the ultrathin copper layer is laminated and hot-pressed to an insulating substrate and then the carrier is peeled via the peeling layer.
Heretofore, it is publicly known that the peeling layer is formed with Cr, Ni, Co, Fe, Mo, Ti, W, P, or their alloys or hydrates. In addition, patent documents disclose that Ni, Fe or their alloy layer is effective as an underlying layer for the peeling layer in order to stabilize the peeling property in a high temperature atmosphere such as hot pressing. (Japanese Patent Application Public Disclosure No. 2010-006071, and Japanese Patent Application Public Disclosure No. 2007-007937).
These publications disclose that uniform plating on the peeling layer is very difficult due to the inherent peeling property of the peeling layer and accordingly there are cases where a great number of pinholes are formed in the ultrathin copper foil depending on the plating conditions. To cope with this problem, the publications disclose that a strike copper layer is first plated on the peeling layer and then copper is plated on the strike copper layer, whereby uniform plating is formed and the number of pin holes in the ultrathin copper foil can be remarkably reduced.